Fluidic oscillators are of considerable interest for metering the volumes of fluids such as water and gas delivered to users. Most flow meters presently in existence use mechanical moving parts. This applies in particular to flow meters using a turbine or a membrane. In comparison, fluidic oscillators do not have any moving parts liable to wear over time, and consequently such oscillators do not need recalibrating.
Such oscillators can be small in size and very simple in design. They are therefore very reliable. In addition they deliver a frequency signal which is easily converted into a digital signal. This characteristic is particularly advantageous when meters are read remotely.
Most effort in developing such flow meters has been directed to vortex effect flow meters and to Coanda effect flow meters.
The principle on which vortex effect flow meters operate is based on the well-known fact that the presence of an obstacle in a duct along which a fluid is flowing gives rise to eddies that escape periodically. Measurement is based on detecting the frequency at which eddies detach, which frequency is proportional to the flow speed for an obstacle of given geometry.
The eddy frequency is measured in various ways which make it possible to derive the mean speed of the flow and thus the flow rate. Vortex effect flow meters are generally very sensitive to noise and to fluid conditions upstream. In practice, a flow rectifier is used to make the speed profile uniform. A flow meter of this type is described, for example, in U.S. Pat. No. 3,589,185.
The Coanda effect as used in Coanda effect flow meters consists in the natural tendency of a fluid jet to follow the contours of a wall when the jet is discharged close to the wall, even if the outline of the wall departs from the discharge axis of the jet. A fluidic oscillator of this type includes a chamber into which the fluid jet discharges through a converging nozzle. Two side walls are placed in the chamber symmetrically about the discharge axis of the jet. The jet leaving the inlet to the oscillator attaches itself spontaneously to one of the side walls by the Coanda effect. A portion of the flow is then bled off via a lateral channel of the wall to which the jet attaches itself, thereby having the effect of detaching the jet from said wall and attaching it to the opposite wall. The phenomenon then takes place again, thus giving rise to permanent oscillation in the incoming flow. Unfortunately, in this type of apparatus, the range over which flow can be measured is relatively limited and the nonlinearity of the calibration curve is quite large. Furthermore, this type of apparatus may stop oscillating under certain conditions relating to external disturbances, and this gives rise to a loss of signal. To increase the range in which measurement is possible, Okadayashi et al. have proposed, in U.S. Pat. No. 4,610,162, combining two fluidic oscillators, one operating at low flow rates and the other at high flow rates.
Because of the drawbacks encountered with vortex effect and with Coanda effect flow meters, attempts have been made to develop other types of fluidic oscillator which operate using principles that are fundamentally different. Applications thereof are found in the flow meters described in the following U.S. Pat. Nos.: 4,184,636, 4,244,230 and 4,843,889.
For example, U.S. Pat. No. 4,244,230 describes a fluidic oscillator flow meter placed in a duct on the path of the fluid, and extracting a portion of the fluid. The oscillator has two members disposed side by side and having facing walls that form a nozzle. An obstacle has a frontal oscillator chamber placed facing the nozzle. The chamber has a common inlet and outlet. The jet leaving the nozzle penetrates into the chamber and strikes the far wall of the chamber.
The jet is put into transverse oscillation inside the chamber by the formation of two eddies on either side of the jet. The eddies alternate between being strong and weak, in phase opposition. The jet leaves via the common outlet and is directed into the main flow.
Pressure sensors enable the oscillation frequency of the jet in the chamber to be measured, which frequency is proportional to the flow rate.
The performance of flow meters of that type is generally better than the performance obtained using conventional fluidic flow meters. Unfortunately, said performance is still not satisfactory, in particular with respect to sensitivity and measurement range.
An object of the present invention is to remedy the above drawbacks. The invention provides a fluidic oscillator and a flow meter including such an oscillator and having performance that is better than that of prior art flow meters.